1. Field of the Invention
The present invention relates to a high DC gain wideband operational amplifier operable at a low operating voltage.
2. Description of the Related Art
A prior art operational amplifier is constructed by a drive cascode circuit connected to output terminals including a differential pair for receiving differential input voltages, a tail current source connected between the differential pair and a ground terminal, and a load cascode circuit connected between a power supply terminal and the output terminals for generating constant load currents (see: Germano Nicollini et al., “A Fully Differential Sample-and-Hold Circuit for High-Speed Applications”, IEEE Journal of Solid-State Circuits, Vol. 24, No. 5, pp. 1461–1465, October 1989). This will be explained later in detail.
In the above-described prior art operational amplifier, however, a high DC gain cannot be realized, and also the minimum operating voltage is relatively high so that the power consumption would be increased.
Note that, in order to increase the DC gain, triple cascode circuits may be used instead of the above-mentioned cascode circuits; in this case, however, the dynamic output range would be decreased. Also, in order to increase the DC gain, gain boost circuits can be connected to the above-mentioned cascode circuits; in this case, however, the integration would be degraded and the power consumption would be increased. Regarding “gain boost circuits”, refer to T. B. Chuo et al., FIG. 6 of “A 10b, 20 Msample/s, 35 mW Pipeline A/D converter”, IEEE Journal of Solid-State Circuits, Vol. 30, No. 3, pp. 166–172, March 1995.